ARQ in a point to multipoint network

ABSTRACT

The systems and methods for incorporating an automatic retransmission mechanism into a point to multipoint network MAC layer that does not already incorporate retransmission are provided by virtue of one embodiment of the present invention. For example, the DOCSIS MAC layer protocol developed for data over cable networks may be modified to incorporate an automatic retransmission mechanism. This allows DOCSIS to be applied more effectively to communication channels such as the wireless communication channel.

BACKGROUND OF THE INVENTION

The present invention is relates to data communications and moreparticularly to systems and methods for improving the performance ofpoint to multipoint networks.

A point to multipoint wireless communication system represents apotentially effective solution to the problem of providing broadbandnetwork connectivity to a large number of geographically distributedpoints. Unlike optical fiber, DSL and cable modems, there is no need toeither construct a new wired infrastructure or substantially modify awired infrastructure that has been constructed for a different purpose.In a typical wireless point to multipoint network design, there is acentral access point that provides connectivity to the Internet backboneand numerous subscriber units that interact directly with the centralaccess point. Communication from the head end or central access point tothe individual subscriber units is referred to as downstreamcommunication. Communication from a subscriber unit to the centralaccess point is referred to as upstream communication.

In one scenario, upstream and downstream communications use differentfrequencies. A time division multiple access (TDMA) scheme may be usedto divide access to the upstream communication channel among the varioussubscriber units.

Except for the physical medium, the wireless point to multipoint networkarchitecture just described is in fact very similar to the architectureof a cable modem network as defined by the DOCSIS standard promulgatedby Cable Television Laboratories, Inc. In order to take advantage of thelarge body of experience in operating cable modem networks and furtherto be able to employ components already developed for cable modemnetworks, it is desirable to adapt at least the media access contention(MAC) layer portion of the DOCSIS protocol to the wireless point tomultipoint network while employing a physical layer that is suitable forwireless applications. The physical wireless communication channelgenerally represents a greater challenge than the cable channel due tonoise, interference and greater multipath effects. Adapting DOCSIS tothe wireless environment involves in part using a more robust physicallayer such as one based on OFDM (orthogonal frequency divisionmultiplexing).

Network protocols designed from the start for wireless communicationoften include a mechanism referred to as “ARQ” that provides foracknowledgment of successfully transmitted information andretransmission where there is no such acknowledgment. ARQ increases theperformance of wireless communication networks in challenging channelconditions because the odds of successive transmission are greatlyincreased if multiple attempts are permitted. Unfortunately, DOCSISmakes no provision for ARQ in its MAC layer design. Furthermore, it isvery cumbersome to include ARQ at the physical layer and still takeadvantage of DOCSIS MAC layer components. What is needed are systems andmethods for adapting ARQ to the DOCSIS MAC layer while making minimalchanges to the DOCSIS protocol.

SUMMARY OF THE INVENTION

Systems and methods for incorporating an automatic retransmissionmechanism into a point to multipoint network MAC layer that does notalready incorporate retransmission are provided by virtue of oneembodiment of the present invention. For example, the DOCSIS MAC layerprotocol developed for data over cable networks may be modified toincorporate an automatic retransmission mechanism. This allows DOCSIS tomore effectively handle challenging communication channels such as thewireless communication channel.

A first aspect of the present invention provides a method of operating asubscriber unit in a point to multipoint network operating according toa DOCSIS-based MAC protocol. The method includes transmitting data to acentral access point during a directed grant slot allocated to thesubscriber unit, monitoring MAP messages broadcast by the central accesspoint to detect acknowledgment of receipt of the data, and if noacknowledgement of receipt is indicated by the MAP messages,retransmitting the data.

A second aspect of the present invention provides a method of operatinga central access point in a point to multipoint network operatingaccording to a DOCSIS-based MAC protocol. The method includes receivingdata from a subscriber unit during a directed grant slot allocated tothe subscriber unit and broadcasting a MAP message that includes anacknowledgment of receipt of the data.

Further understanding of the nature and advantages of the inventionherein may be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a point to multipoint network according to one embodimentof the present invention.

FIG. 2 depicts a subscriber unit according to one embodiment of thepresent invention.

FIG. 3 depicts a central access point according to one embodiment of thepresent invention.

FIG. 4 depicts message flow according to one embodiment of the presentinvention.

FIG. 5 is a flowchart describing steps of operating an ARQ scheme in asubscriber unit according to one embodiment of the present invention.

FIG. 6 is a flowchart of operating an ARQ scheme in a central accesspoint according to one embodiment.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 depicts a point to multipoint wireless communication network 100suitable for implementing one embodiment of the present invention. Thepresent invention is, however, not limited to wireless networks. Network100 includes a central access point or head end 102 and multiplesubscriber units 104. All communication is typically either to or fromcentral access point 102. Communication from central access point 102 toone or more subscriber units 104 is herein referred to as downstreamcommunication. Communication from any one of subscriber units 104 tocentral access point 102 is herein referred to as upstreamcommunication. In one embodiment, different frequencies are allocated toupstream and downstream communication. This is referred to as frequencydivision duplexing (FDD). Alternatively, time division duplexing (TDD)may be employed where upstream and downstream communications share thesame frequencies.

Each of one or more upstream frequencies is common to multiplesubscriber units. A media access control (MAC) protocol is provided tocontrol access by the subscriber units to the shared medium. One type ofsuch protocol is referred to as time division multiple access (TDMA). Ina TDMA protocol, the time domain is divided into frames or slots.

An example of a TDMA MAC protocol is the DOCSIS v1.1 standard which wasdeveloped for data over cable networks. The DOCSIS v1.1 standard isdefined by the Data-Over-Cable Service Interface Specifications, RadioFrequency Interface Specification SP-RFIv 1.1-I06-001215 published bythe Cable Television Laboratories, Inc. in 2000, the contents of whichare incorporated herein by reference in their entirety.

Among the types of upstream transmission slots supported by DOCSIS are“request/data” slots and “data grant” slots. The data grant slots arereserved for particular subscriber units and are referred to herein as“directed” grant slots. During any directed grant slot only onesubscriber unit may transmit. Multiple subscriber units are permitted totransmit during a request/data slot. A collision may result fromsimultaneous transmission by multiple subscriber units so DOCSISprovides that upstream transmissions during request/data slots areacknowledged so that a subscriber unit knows that it need not repeat itstransmission. Typically, the request/data slot is used to request areserved data grant slot rather than for upstream transmission of higherlayer data.

In order to coordinate the upstream transmissions of subscriber units,the central access point periodically transmits scheduling messagesdownstream that are referred to as “MAP” messages. A MAP messageincludes a series of information elements that identify and characterizeupstream time slots. The information element for a time slot willidentify the type of time slot (request/data, directed grant, etc.), theprecise time of the time slot and for time slots reserved for particularsubscriber units, an identifier identifying the subscriber unit and MAClayer service on that subscriber unit for which the time slot has beenreserved. The MAP messages also include acknowledgments of data packetsreceived during any request/data time slots since the previous MAPmessage, as well as a time stamp indicative of the latest time slot forwhich successful receipt of a data packet during that time slot willhave been acknowledged.

In order to improve the robustness of the DOCSIS MAC layer underdifficult physical layer channel conditions, it is desirable to providea mechanism for retransmitting data packets that have beenunsuccessfully transmitted in directed grant slots. DOCSIS provides noacknowledgment mechanism for such unsuccessfully transmitted packets.According to one embodiment of the present invention, an acknowledgmentand automatic retransmission mechanism may be added to DOCSIS v1.1 byhaving central access point 102 acknowledge successfully receivedpackets in directed grant slots using the same acknowledgement mechanismprovided for data transmitted upstream during request/data slots.

The DOCSIS MAC layer entity at central access point 102 is modified sothat acknowledgments for the packets received during directed grantslots are included in the MAP message. The subscriber unit DOCSIS MAClayer entity is modified so that data transmitted upstream duringdirected grant slots is stored after transmission and retransmitted ifno acknowledgment is received prior to receipt of a MAP message having atime stamp later than the directed grant slot used for the initialtransmission. This acknowledgement and retransmission or ARQ mechanismis preferably applied only to data traffic that can tolerate the greaterlatency inherent in retransmission. Thus, ARQ might not be applied toreal time data streams such as voice and/or streaming video traffic.

FIG. 2 depicts central access point 102 according to one embodiment ofthe present invention. An antenna 202 picks up and sends signals via theairwaves. A physical layer transceiver 204 acts as a wireless modem.Physical layer transceiver 204 includes analog functionality such asamplification, filtering, conversion between the radio frequency (RF)used for transmission and reception over the airwaves and anintermediate frequency (IF), conversion between the IF and base band,conversion between analog and digital signals, digital filtering, errorcorrection coding and decoding, conversion between bits and data symbolsused for communication, interleaving and deinterleaving, etc. Physicallayer transceiver 204 may also include signal processing functionalityto support orthogonal frequency division multiplexing (OFDM) as well assignal processing to take advantage of the use of multiple antennas foroptimal transmission and/or reception. As a part of its error correctiondecoding function, physical layer transceiver 204 can determine if adata packet has been correctly received.

A MAC layer processor 206 implements the central access point MAC layerfunctionality defined by the DOCSIS v1.1 specification cited above asmodified to implement the ARQ capability provided by the presentinvention. MAC layer processor 206 processes access requests, grantstransmission slots, formulates and transmits MAP messages, etc.

MAC layer processor 206 may be implemented entirely in hardware but alsomay rely on instructions from a machine-readable medium for at least apart of its operation. A computer readable storage medium 208 isdepicted and may include program instructions, state information, etc.for performing any of the functions described herein. Examples ofmachine readable media include, but are not limited to, magnetic mediasuch as hard disks, floppy disks, and magnetic tape; optical media suchas CD-ROM disks; magneto-optical media such as optical disks; andhardware devices specially configured to store and program instructions,such as read only memory devices (ROM) and random access memory (RAM).Another example of a computer-readable storage medium is a carrier wavetraveling an appropriate medium such as airwaves, optical lines,electrical lines, etc. Examples of program instructions include bothmachine codes such as produced by a compiler and files containing ahigher-level code that may be executed using an interpreter.

FIG. 3 depicts subscriber unit 104 according to one embodiment of thepresent invention. Signals are exchanged with the airwaves via antenna302. A physical layer transceiver 304 performs functions analogous tothose of physical layer transceiver 204 within central access point 102.A MAC layer processor 306 performs the DOCSIS v1.1 MAC layerfunctionality substantially as specified for subscriber units andmodified to incorporate the ARQ functionality provided by the presentinvention. An ARQ buffer 308 is provided to store data packets that havealready been transmitted to allow for retransmission when noacknowledgement is received. A storage medium 310 may store instructionsfor the operation of MAC layer processor 306 and may be implemented inany of the ways described with reference to storage medium 208.

To facilitate the description of ARQ operation in the DOCSIS context, itwill be helpful to briefly explain the concept of DOCSIS service flows.To support streaming applications, DOCSIS provides for multiple “serviceflows” where each service flow is a unidirectional flow of packets thatis provided a particular quality of service. For example, at aparticular subscriber unit, a first upstream service flow may beemployed for web-related data while a second upstream service flow maybe used for a voice call. Each service flow has an associated subscriberidentifier (SID) value. When allocating time slots, central access point102 specifies the service flow that should transmit upstream during eachallocated time slot.

FIG. 4 depicts upstream and downstream message flow according to oneembodiment of the present invention. The top horizontal axis of FIG. 4depicts representative downstream messages. The bottom horizontal axisdepicts representative upstream messages. The horizontal dimensioncorresponds to time but the width of individual time slots is not drawnto scale with the spacing between the slots.

FIG. 4 depicts a scenario where a subscriber unit 104 transmits a datapacket upstream to the central access point, the data is not receivedsuccessfully by the central access point, and the subscriber unitretransmits after noting the missing acknowledgment. A data/request timeslot is used to transmit an access request 402 upstream from thesubscriber unit 104 to central access point 102. In response, centralaccess point 102 in its next MAP message 404 includes an informationelement identifying a directed grant slot reserved for the requestingsubscriber unit 104. The subscriber unit 104 transmits the data packetupstream in message 406 during the reserved directed grant slot. Due toa channel impairment such as noise and/or interference, this data packetis not successfully received by the central access point. Accordingly, aMAP message 408 following the directed grant slot does not include anacknowledgment of successful receipt and has a time stamp that is laterthan the directed grant time slot.

Based on an inspection of the contents of MAP message 408, thesubscriber unit 104 recognizes that message 406 has not beensuccessfully received. Accordingly, it again requests access in amessage 410. Central access point 102 responds by including aninformation element scheduling a new directed grant slot in a MAPmessage 412. Then the data packet is retransmitted in a message 414 thatoccupies the reserved directed grant slot.

FIG. 5 is a flow chart describing steps of implementing an ARQ scheme ina subscriber unit according to one embodiment of the present invention.At step 502, a service flow operating on subscriber unit 104 requestsaccess to transmit data upstream. At step 504, this service flowreceives a MAP message assigning it a directed grant slot to transmitthis data upstream. Then, at step 506, the subscriber unit 104 sends thepacket belonging to this service flow upstream during the directed grantslot.

ARQ is preferably implemented only for data traffic that is tolerant ofdelay or latency. At a step 508 the subscriber unit determines if thetransmitted data packet qualifies for ARQ. For example, a packetcarrying voice over IP traffic will preferably not qualify since if sucha packet is lost during transmission it would be less detrimental tovoice quality to simply skip the lost packet rather than requestretransmission. Thus if the packet does not qualify for ARQ, special ARQhandling terminates at step 510 and normal DOCSIS processing resumes. Ifthe packet does qualify for ARQ, then it is stored in ARQ buffer 308 ata step 512.

Now subscriber unit 104 begins monitoring MAP messages to check foracknowledgment for successful receipt of the packets sent upstream instep 506. At a step 514, the subscriber unit 104 receives a MAP message.The subscriber unit 104 inspects the MAP message for acknowledgment ofsuccessful receipt of the data packet at step 516. If the MAP messageincludes this particular acknowledgement, this is another basis fortermination of the ARQ procedure at a step 518. If the MAP message doesnot include an acknowledgment, then the subscriber unit 104 tests, at astep 520, if the time stamp of the MAP message indicates a time afterthe directed grant slot used to transmit the data packet. If this MAPmessage time stamp is before the time of the directed grant slot, thereis a possibility that a later MAP message will include theacknowledgment and ARQ processing returns to step 514. If the MAP timestamp is after the directed grant slot, this means that the data packetwas not successfully received, and the subscriber unit retransmits atstep 522 after retrieving the data packet from ARQ buffer 308.

FIG. 6 is a flow chart describing steps of operating an ARQ scheme incentral access point 102 according to one embodiment of the presentinvention. At step 602, central access point 102 receives an accessrequest from subscriber unit 104 for time to be reserved to transmitdata upstream. The access request originates with a particular serviceflow and includes the SID of that service flow. At step 604, the centralaccess point sends a MAP message that includes an information elementidentifying a directed grant slot reserved for upstream datatransmission by the requesting service flow.

Once the directed grant time slot is reached, a step 606 tests whetherdata was successfully received from the requesting subscriber unit 104and whether this slot was in fact allocated to a service flow that sendsdata that qualifies for ARQ. For example, if the data packet belongs toa service flow supporting a voice call it will preferably not qualifyfor ARQ. If the data was not received during the directed grant slotsuccessfully, or the slot was allocated to data not qualifying for ARQ,then standard DOCSIS operation continues at step 608. If the data wassuccessfully received in the directed grant slot and the data qualifiesfor ARQ, then acknowledgment of successful receipt is included in thenext MAP message.

It can be seen that an ARQ mechanism has been incorporated into DOCSISv1.1 with minimal change to the protocol. This provides greaterfunctionality in difficult communication channels including wirelesschannels suffering impairment such as noise and/or interference.

It is understood that the examples and embodiments that are describedherein are for illustrative purposes only and various modifications arechanges in light there of will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication and scope of the appended claims and their full scope ofequivalents. For example, the present invention may be applied tosystems exploiting MAC protocols other than DOCSIS and into networksother then wireless networks. All publications, patents, and patentapplications cited herein are hereby incorporated by reference.

1. In a point to multipoint network operating according to aDOCSIS-based MAC protocol, a method for operating a subscriber unit,said method comprising: transmitting data to a central access pointduring a directed grant slot allocated to said subscriber unit;determining if said transmitted data qualifies for ARQ; and if saidtransmitted data qualifies for ARQ; monitoring MAP messages broadcast bysaid central access point to detect acknowledgement of receipt of saiddata transmitted during said directed grant slot; and if noacknowledgement of receipt of said data transmitted during said directedgrant slot is indicated by said MAP messages, retransmitting said data;wherein non-receipt is indicated by said MAP messages if a MAP messageis received with a timestamp later than said directed grant slot and noacknowledgement is received.
 2. The method of claim 1 furthercomprising: after transmitting said data to said central access pointduring said directed grant slot, storing said data in an ARQ buffer forpossible retransmission.
 3. The method of claim 2 wherein storing saiddata comprises storing said data only if communication of said data isdelay tolerant.
 4. The method of claim 1 further comprising determiningif a time stamp of said MAP message indicates a time after said directedgrant slot.
 5. The method of claim 4 wherein said data is retransmittedonly if said MAP time stamp indicates a time after said directed grantslot.
 6. In a point to multipoint network operating according to aDOCSIS-based MAC protocol, a method for operating a central accesspoint, said method comprising: receiving data from a subscriber unitduring a directed grant slot allocated to said subscriber unit; andbroadcasting a MAP message including an acknowledgment of receipt ofsaid data received during said directed grant slot and a time stampindicative of a latest time slot for which successful receipt of a datapacket during that time slot was acknowledged, if said directed grantslot is allocated to data that qualifies for ARQ.
 7. The method of claim6 wherein said data qualifies for ARQ if said data is delay tolerant. 8.The method of claim 6 further comprising: prior to said directed grantslot, broadcasting another MAP message allocating said directed grantslot to said subscriber unit.
 9. The method of claim 6 furthercomprising receiving an access request from a subscriber unit for timeto be reserved to transmit data, and wherein said access requestcomprises a subscriber identifier value associated with a service flowof said data.
 10. The method of claim 9 further comprising sending a MAPmessage comprising an information element identifying a directed grantslot reserved for upstream data transmission.
 11. The method of claim 6wherein broadcasting a MAP message comprises automatically broadcastinga MAP message each time data is received during said directed grantslot.
 12. In a point to multipoint network operating according to aDOCSIS-based MAC protocol, apparatus for operating a subscriber unit,said apparatus comprising: a MAC layer processor that transmits data toa central access point during a directed grant slot allocated to saidsubscriber unit, monitors MAP messages broadcast by said central accesspoint to detect acknowledgment of receipt of said data transmittedduring said directed grant slot, and determines if said transmitted dataqualifies for ARQ; and an ARQ buffer that stores said data transmittedduring said directed grant slot after it is transmitted if saidtransmitted data qualifies for ARQ; and wherein if no acknowledgment ofreceipt is indicated by said MAP messages, said MAC layer processorretrieves said data from said ARQ buffer and retransmits said data;wherein non-receipt is indicated by said MAP messages if a MAP messageis received with a timestamp later than said directed grant slot and noacknowledgement is received.
 13. The apparatus of claim 12 wherein saidARQ buffer stores said data only if communication of said data is delaytolerant.
 14. In a point to multipoint network operating according to aDOCSIS-based MAC protocol, apparatus for operating a central accesspoint, said apparatus comprising: a physical layer transceiver thatexchanges information signals with a subscriber unit via a transmissionmedium; and a MAC layer processor that receives data during a directedgrant slot allocated to said subscriber unit via said physical layertransceiver and that broadcasts a MAP message including anacknowledgment of receipt of said data received during said directedgrant slot and a time stamp indicative of a latest time slot for whichsuccessful receipt of a data packet during that time slot wasacknowledged, if said directed grant slot is allocated to data thatqualifies for ARQ.
 15. The apparatus of claim 14 wherein saidacknowledgment is included only if said data is delay tolerant.
 16. Theapparatus of claim 14 wherein said MAC layer processor, prior to saiddirected grant slot, broadcasts another MAP message allocating saiddirected grant slot to said subscriber unit.
 17. In a point tomultipoint network operating according to a DOCSIS-based MAC protocol,apparatus for operating a subscriber unit, said apparatus comprising:means for transmitting data to a central access point during a directedgrant slot allocated to said subscriber unit; means for determining ifsaid transmitted data qualifies for ARQ; and means for monitoring MAPmessages broadcast by said central access point to detect acknowledgmentof receipt of said data transmitted during said directed grant slot; andmeans for, if no acknowledgment of receipt is indicated by said MAPmessages, retransmitting said data, if said transmitted data qualifiesfor ARQ; wherein non-receipt is indicated by said MAP messages if a MAPmessage is received with a timestamp later than said directed grant slotand no acknowledgement is received.
 18. In a point to multipoint networkoperating according to a DOCSIS-based MAC protocol, apparatus foroperating a central access point, said apparatus comprising: means forreceiving data from a subscriber unit during a directed grant slotallocated to said subscriber unit; and means for broadcasting a MAPmessage including an acknowledgment of receipt of said data receivedduring said directed grant slot and a time stamp indicative of a latesttime slot for which successful receipt of a data packet during that timeslot was acknowledged, if said directed grant slot is allocated to datathat qualifies for ARQ.
 19. In a point to multipoint network operatingaccording to a DOCSIS-based MAC protocol, a computer program product foroperating a subscriber unit, said computer program product comprising:code that causes transmission of data to a central access pint during adirected grant slot allocated to said subscriber unit; code thatdetermines if said transmitted data qualifies for ARQ; code that causesmonitoring of MAP messages broadcast by said central access point todetect acknowledgment of receipt of said data transmitted during saiddirected grant slot; code that causes, if no acknowledgment of receiptis indicated by said MAP messages, retransmission of said data if saidtransmitted data qualifies for ARQ; and a computer readable medium thatstores the codes; wherein non-receipt is indicated by said MAP messagesif a MAP message is received with a timestamp later than said directedgrant slot and no acknowledgment is received.
 20. The computer programproduct of claim 19 further comprising: code that causes, aftertransmission of said data to said central access point, storing of saiddata in an ARQ buffer for possible retransmission.
 21. The computerprogram product of claim 20 wherein said code that causes storing ofsaid data causes storing of said data only if communication of said datais delay tolerant.
 22. In a point to multipoint network operatingaccording to a DOCSIS-based MAC protocol, a computer program product foroperating a central access point, said computer program productcomprising: code that causes reception of data from a subscriber unitduring a directed grant slot allocated to said subscriber unit; codethat causes broadcasting a MAP message including an acknowledgment ofreceipt of said data received during said directed grant slot and a timestamp indicative of a latest time slot for which successful receipt of adata packet during that time slot was acknowledged, if said directedgrant slot is allocated to data that qualifies for ARQ; and acomputer-readable storage medium that stores the codes.
 23. The computerprogram product of claim 22 wherein said acknowledgment is included onlyif said data is delay tolerant.
 24. The computer program product ofclaim 22 further comprising: code that, prior to said directed grantslot, causes broadcasting of another MAP message allocating saiddirected grant slot to said subscriber unit.
 25. In a point tomultipoint network operating according to a DOCSIS-based MAC protocol, amethod for operating a subscriber unit, said method comprising:transmitting an access request to a central access point on arequest/data slot, the access request being arranged to request accessto transmit data upstream; receiving a first MAP message in response tothe access request, the first MAP message being arranged to allocate adirected grant slot to the subscriber unit, the directed grant slotbeing arranged such that substantially only the subscriber unit maytransmit during the directed grant slot; transmitting the data to thecentral access point during the directed grant slot allocated to thesubscriber unit; determining if said transmitted data qualifies for ARQ;and if said transmitted data qualifies for ARQ; monitoring a pluralityof MAP messages broadcast by the central access point to detectacknowledgment of receipt of the data transmitted to the central accesspoint during the directed grant slot; and when no acknowledgment ofreceipt of the data transmitted during the directed grant slot isindicated by the plurality of MAP messages, retransmitting the datatransmitted to the central access point during the directed grant slot;wherein non-receipt is indicated by said MAP messages if a MAP messageis received with a timestamp later than said directed grant slot and noacknowledgement is received.
 26. The method of claim 25 furtherincluding: determining when to store the data transmitted to the centralaccess point during the directed grant slot; and storing the datatransmitted to the central access point during the directed grant slotwhen it is determined that the data transmitted to the central accesspoint during the directed grant slot is to be stored, where the datatransmitted to the central access point during the directed grant slotis retransmitted when the data transmitted to the central access pointduring the directed grant slot is stored and when there is noacknowledgement of receipt of the data transmitted during the directedgrant slot.